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One of the most common fuel related failures we have witnessed on the LLY Duramax is that of the fuel control actuator. Basically, the FCA is the metering device for the CP3 injection pump.
If the FCA is not working properly, the ECM cannot communicate to the pump how much fuel it should be delivering. In most cases, people first know there is an issue when they have a hard start or no start issue because the FCA will not allow the pump to properly build pressure to start. However, by the time this occurs, the truck has probably been delivering less than the truly desired amount of fueling for a while already as the FCA is a simple spring design that many times takes a rather slow death, and not an immediate failure.
If you're experiencing hard start or no start issues, one easy way to confirm whether or not the FCA is causing the issue is to simply unplug it, thus eliminating any metering. Presuming the truck starts, immediately turn it off as you will have full pressure running to the fuel rail, but at least you have fairly high certainty that the FCA was causing the issue.